1. Field of the Invention
The invention relates generally to Global Positioning System (GPS) receivers and more particularly to a GPS receiver system wherein a GPS Smart Antenna, a differential GPS radio receiver, and a personal computing device are coupled by an infrared link.
2. Description of the Prior An
GPS receivers are now used for many applications to provide a geographical location. The GPS receiver includes a GPS antenna to receive a GPS signal transmitted from one or more GPS satellites, a GPS engine to compute the geographical location of the antenna and the time of observation of that location, a display processor to convert the location and observation time into information that is useful for an application, and a display device to show the information to the user. The antenna must be positioned with a direct line of sight to the satellite or satellites from which the signals are received.
An important figure of merit in a GPS receiver is accuracy of the geographical location. The inherent accuracy of the GPS location measured by a commercial GPS receiver is approximately 20 meters. However, the United States Government currently employs selective availability (SA) to degrade the accuracy of the GPS location that is determined by a commercial GPS receiver. With SA the GPS location accuracy is approximately 100 meters. Several applications require a geographical location accuracy that is better than 100 meters or even 20 meters. For example, a 100 to 20 meter location error could lead to unintentional trespassing, make the return to an underground marker or mineral difficult, place a motor vehicle on the wrong block, or cause a navigator to choose an incorrect course for a boat or an airplane.
Fortunately, both the inherent and the SA-degraded GPS location accuracy can be improved by the application of differential GPS (DGPS) corrections. In general, the DGPS corrections are derived by taking the difference between a GPS location determined by a GPS receiver located at a reference site and a surveyed location of the reference site. Various airwave radio frequency signals are now available from a variety of sources to provide the DGPS corrections in real time to a mobile GPS receiver system. A DGPS radio receiver included as a part of a GPS receiver system receives the airwave signal carrying the DGPS corrections. The mobile GPS receiver system uses the DGPS corrections to correct the GPS location. The corrected GPS location, termed a "DGPS location" has an accuracy in a range of 10 constructions for the DGPS radio receiver are required depending upon which of the various airwave signals the DGPS radio receiver is to receive.
Another important figure of merit for a GPS receiver is portable computing power. In many applications, the GPS location or the DGPS location is processed to provide further information that is useful to a user. For example, a geographic information system (GIS) application may store the geographical locations and attributes of map features in the form of an electronic map. A navigation application may need to compute a distance and a direction to a selected map feature or to a map feature having a selected set of the attributes. Such applications require a large memory and are most expediently programmed in a processing system that has the power to run a standard operating system, such as DOS, DOS with Windows, Macintosh, GeoWorks, and others. Fortunately, personal computing devices have recently become available that have the portability, memory, and processing power to run these applications.
Several formats of GPS receiver systems exist or have been proposed that include system components of the GPS receiver, the DGPS radio receiver, and the personal computing device in order to provide DGPS location capability, portability, and processing power. In a first format, the system components are integrated into a single unit. Such units may be "hardwired" into a single unit, or the GPS receiver and the DGPS radio receiver may be housed on Personal Computer Memory Card Interface Association (PCMCIA) cards that plug into the personal computing device to give the effect of a single unit. An advantage of using PCMCIA cards, is that the various constructions of DGPS radio receivers do not prevent the manufacture of a standard construction of the GPS receiver on a separate card. A problem with this format is that the user must remain in the open to preserve a direct line of sight from the GPS antenna to one or more GPS satellites while operating and observing the personal computing device. One solution to this problem is to place the GPS antenna in a separate unit, connected with the GPS receiver system by a cable. In a second format, the system components are each housed in separate units and interconnected with cables. This format retains the advantages of separating the GPS antenna from the system and of having a standardized construction of the GPS receiver. However, cables and their connections are expensive, prone to breakage or malfunction, and inconvenient for some applications.
In a third format, the GPS receiver and the DGPS radio receiver components are integrated into a GPS/DGPS Smart Antenna unit. The GPS/DGPS Smart Antenna unit may use a wireless radio frequency or infrared (IR) frequency link to connect to the personal computing device. The IR frequency link has the advantage that it does not interfere with reception of airwave radio frequency signals used for navigation and does not require testing or certification by the FAA or FCC. This format eliminates the expense, reliability problems, and inconvenience of the cable but, does not allow a standard construction of the GPS receiver component.
There is a need for a GPS receiver system to provide a geographical DGPS location, where the system includes a GPS receiver having a standard construction and where the system components are interconnected by an infrared (IR) link.